Following their 2-0 win in the opening weekend over newly-promoted Real Oviedo, Villarreal got off to a flyer at home again, scoring twice inside the opening 16 minutes.
The first came in the 7th minute when Nicolas Pepe capitalised on a defensive mistake before composing himself to slide the ball past Vladyslav Krapyvtsov.
Then in the 16th minute, the Yellow Submarine were two goals to the good following a moment of pure individual brilliance from Buchanan.
The Canadian international collected the ball on the right flank before showing incredible pace and power to knock the ball into the box and let fly into the roof of the net.
Girona were well and truly on the ropes, and just as it looked like they settled into the contest, the home side went and added a third and fourth goal in a three-minute blitz.
On 25 minutes, a short corner routine culminated with Yeremy Pino whipping a ball to the far post, where Rafa Marin nodded in.
The Yellow Submarine rounded off a dominant spell in the 28th minute as Sergi Cardona slid a ball into the box for Buchanan to prod home.
Four goals to the good, the home side coasted through the early moments of the second half and had little reason to press forward.
The away side, on the other hand, were clearly defeated and were already looking to the full-time whistle.
Their misery was further compounded on 64 minutes as Buchanan rounded off a stunning evening and a hat-trick to go with it. Released down the right following a counterattack, the Canadian powered the ball into the back of the net.
The final half hour was not far off a training exercise for Marcelino’s men, who have got their season off to a flyer, winning two in two while scoring seven and conceding none.
Girona, on the other hand, are still suffering a fall from grace following their top-four finish two seasons ago, with the Catalan side losing their two games this season.
Flashscore Man of the Match: Tajon Buchanan (Villarreal)